A routing device performs layer 3 (L3) communication over different network segments through two steps:
1. Search a routing table on the current routing device according to a DIP (destination Internet Protocol, destination IP) address, match an NH (Next Hop, next-hop device) of a destination network segment, and match a default route if no route can be matched in the routing table.
2. Search a local ARP (Address Resolution Protocol, address resolution protocol) table on the current routing device according to the NH, and match a precise ARP entry; and if no precise ARP entry exists, send an ARP request to the NH, and after receiving a response message of the NH, learn and add an ARP entry of the NH to the ARP table for later query, and forward an IP packet on an Ethernet link after the routing device performs Ethernet header encapsulation for the IP packet according to the learned ARP entry.
If the current routing device has the above entries, the current routing device can successfully forward the IP packet to the NH, that is, the routing device must have an NH route to the destination network segment and an ARP entry of the NH, and the IP packet can be successfully sent to the NH on the link and then to a destination device only when the two conditions are satisfied. In the layer 3 forwarding process, the ARP entry of the NH needs to be learned before the IP packet can be forwarded. A delay occurs inevitably at the first time of packet forwarding. For a packet sensitive to the delay, for example, a voice packet, the delay in forwarding the packet affects the call quality, and if the ARP entry of any node on the forwarding path fails to be learned, for example, because an ARP entry conflict occurs or the ARP table is full, the service is interrupted because of packet loss.